In most movies and video games I have observed people mentioning ammo type and capacity such as, 5.56, 7.76, 9mm, 0.50 calibrate, .45 ACP.
What are these ammo type ?
Edit1: 0.50 Calibre, my mistake!
In: Engineering
Calibre is measured in inches traditionally but a lot of cartridges (bullet plus brass, primer, and smokeless powder, ie the bit you put into the gun) have metric designations. For example, 5.56x45mm is NATO’s go-to intermediate cartridge and came from the .223 Remington, and 7.62x51mm is NATO’s most common battle rifle cartridge and evolved from .30-06 (thirty-aught-six) Springfield. There’s subtle differences as the military cartridges were developed for man-sized targets and so they aren’t 1-to-1 copies of their sporting ancestors today.
9×19 Parabellum is a common pistol cartridge, as is .45 ACP (also called .45 Auto or .45 Browning, but is not to be confused with .45 Smith & Wesson or .45 Colt which is weird because the “C” in .45 ACP does stand for Colt).
.50 doesn’t really mean anything on its own, however it’s commonly applied to .50 BMG (Browning Machine Gun) which is the common NATO heavy machine gun cartridge used by the M2 Browning, however there’s also the 12.7x108mm used by former Soviet countries which is also called a .50 occasionally. .50 has become a byword for any similarly sized cartridge and doesn’t actually refer to a single cartridge.
Simply put, they’re different brands for different purposes.
ELI a bit older than 5:
Now it’s important to note that what cartridge is used in what firearm and the dimensions of the cartridge depend on many different factors. A lever action firearm chambered in .357 S&W Magnum will happily fire a .38 S&W Special, even though the two cartridges clearly have different dimensions (the .38 casing is slightly longer and wider, however because a lever action firearm is cycled manually rather than gas blowback a longer case doesn’t cause issues with ejection, and both cartridges are actually .357 of an inch in diameter despite having different names–the .38 actually refers to the diameter of the casing, so don’t go assuming that the number in a cartridge’s name directly refers to the diameter of the bullet, because it often doesn’t.
Within a single cartridge family you can have a lot of different types. Let’s stick to 5.56x45mm because it is by far the most common cartridge most people will experience, either in real life of through media. Now, the 5.56x45mm cartridge used by the US Military is called M855 and this has some very specific features: the bullet itself has a steel penetrator on top of a lead core surrounded by a copper jacket. The steel penetrator is a sharp point (painted green for easy visual identification when loading a magazine, AKA “bombing in”) to puncture armour and cover, the lead core adds mass (lead is very dense giving it more hitting power) and is malleable enough to deform when there is a sudden change of velocity (ie hitting something soft and wet–hollow point ammunition has the steel penetrator removed and the lead core is exposed, causing even more deformation). This deformation helps creates a cavity within the target. The copper jacketing is because a steel bullet is too hard, and will damage the barrel of the firearm (which has twisted groves in it called rifling to give the bullet a spiral spin–next time you throw a dart or a ball put a bit of a spin on it and see just how much this helps), so a softer metal is used instead so that is eroded rather than either the steel penetrator or the barrel. Because there’s different metals used this changes the mass of the bullet (measured in gr for grains, 1gr is 0.064 grammes or 0.002 ounces), so when you buy cartridges you have to consider how many grains the bullet weighs and what it’s composition is, because it could be the difference between eating some tasty venison or combing the woods for deer bolognese, and using military surplus M855 is a guaranteed way to get yourself kicked off a shooting range because those targets are expensive and the owner doesn’t want them blown apart too quickly.
Another consideration is the propellant. Almost all cartridges these days are smokeless powder which is smokeless (funnily enough) and it contains its oxide for combustion (and thus propulsion). Contrary to popular belief you can fire a gun in space because the oxide is already chemically present within the cartridge. Older cartridges use a material called cordite which looks like toffee and is still used in most fireworks. Firearm collectors often struggle to get original cartridges in rare calibres in cordite because it doesn’t keep well (I’ve fired .303 British with 90 year old cordite propellant and it was not a good experience). Black powder is totally obsolete outside of collecting and practical shooting sports if you’re LARPing as a cowboy. Propellant is also measured in grains and again that can change between different brands of cartridge for different purposes but in the same calibre.
That’s half of what makes a firearm work, the other half is the firearm itself. Barrel length, rifling twist ratio, barrel composition and construction, and many other variables impact how a bullet will fly. You’ll see firearms firing the same calibre with different barrel lengths because there is a tradeoff between velocity, impact force, and MOA (minute of angle, AKA accuracy). If the barrel is too short the bullet won’t pick up enough velocity to fly fast and deliver full force of impact, but if the barrel is too long then you introduce friction and the bullet will start to decelerate before it has left the barrel. For 5.56x45mm M855 NATO a 20″ long barrel is regarded as being the sweet spot, but there are many types of bullet and propellants that are designed for specific barrel lengths, and if you use your firearm for plinking (close target shooting) or pest control other barrel lengths like 14.5″ (carbine length) is also popular because high velocity and MOA actually isn’t necessary in a close environment and the shorter barrel length will be easier to store, carry, and manoeuvre. As the majority of shooting occurs at below 600 metre ranges carbine length rifles tend to be the norm, for both sporting and combat.
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